Homopolar machine with shaft axial thrust compensation for reduced thrust bearing wear and noise

ABSTRACT

A homopolar machine produces an axial counter force on the rotating shaft to compensate for the load on the shaft&#39;s thrust bearing to reduce wear and noise and prolong bearing life. The counter force is produced through magnetic interaction between the shaft and the machine&#39;s field coils and is created by changing the current excitation of the field coils, which results in a magnetic flux asymmetry in an inner flux return coupled to the shaft. The homopolar machine may also have a configuration that uses current collectors that maintain substantially constant contact pressure in the presence of high magnetic fields to improve current collector performance. The current collectors are flexible and may be made from either electrically conductive fibers or stacked strips such that they bear up against the armature so that the pressure is maintained by the spring constant of the current collector material. The homopolar machine may also have a configuration where the brushes are oriented so that the current is aligned as much as is practical with the local magnetic field lines so as to reduce the lateral electromagnetic forces on the brushes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part (CIP) of U.S. applicationSer. No. 09/934,803, filed Aug. 22, 2001 now U.S. Pat. No. 6,628,033,which is a Divisional of U.S. application Ser. No. 09/559,240, filedApr. 26, 2000, now U.S. Pat. No. 6,489,700, the full contents of bothapplications are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to direct current machines, and morespecifically to homopolar machines.

Homopolar machines are operated by direct current (DC) and are simple indesign principle. They have been under development for consideration inship propulsion applications because of their high efficiency, compactsize, low weight, and reduced acoustic signature relative to all othermotors.

As is well known in the art, a homopolar machine includes four majorcomponents: armature; stator; field coils; and flux return. The armatureis connected to the machine's shaft and may also be referred to as therotor. The armature typically includes a series of concentric, coppercylinders and is free to rotate in a direction at right angles to themagnetic field lines produced by the field coils. When a voltage isapplied across the armature in the direction of the shaft, electriccurrent flows parallel to the shaft. The current and magnetic fieldinteraction (I×B) results in torque generation and rotation, thusproducing a motor. In contrast, if the armature is driven externally fora generator application, the interaction of the armature rotating atright angles to the magnetic field lines generates a voltage andelectric current.

In both the motor and generator scenarios, current flows along thearmature and to the stationary stator via sliding electrical contactsreferred to herein as current collectors. The current collectors mayalso be commonly referred to as brushes, and may take the form of manymaterials. Such materials include but are not limited to flexiblefibrous copper, flexible copper strips, or as common to most DC motors,rigid material made from graphite or silver-graphite.

The field coils are typically circumferentially continuous in geometryand aligned on the same central axis with respect to each other. Ahomopolar machine always cuts (or crosses as it rotates) magnetic fluxlines of a magnetic field in the same direction due to the interactingarmature and shaft iron being aligned on the same axis. This means thatany point on the rotor always sees the same magnetic field as it rotatesand no difference in magnetic flux or multiple magnetic poles areencountered by conductive elements of the armature as it rotates. Hencethe nomenclature “homopolar machine”.

The flux return is typically comprised of a highly magneticallypermeable material such as iron or steel. The flux return is designedprimarily to limit the undesirable stray magnetic field that radiatesfrom the machine, and therefore, it typically takes the form of astructural housing that surrounds the motor. In addition, the fluxreturn may also be designed to help direct the magnetic field linesproduced by the field coils into the armature interaction region toimprove the machine's flux utilization.

Although conventional rotating machines are in wide use, most haveinherent disadvantages associated with mechanical bearing wear. Forexample, large machines such as generators connected to turbines, aswell as motors used for propulsion, use thrust bearings to react theinduced axial mechanical forces on the shaft and to maintain a stableshaft axial position. The mechanical thrust that is induced on the shaftinduces an axial thrust on the thrust bearing, which causes thrustbearing wear and noise. The mechanical thrust induced on the shaft maybe from many sources. For example, in a generator application theinduced thrust is caused by the axial pressure differential across aconnected turbine, and in a propulsion motor application the inducedthrust is caused by the thrust exerted by the connected propulsor. If ahomopolar machine was used in these applications, it would be prone tothis same type of problem. Ships, submarines and airplanes allexperience thrust bearing forces within their propulsion systems. Theinduced axial mechanical forces are also referred to herein as thrustbearing loads.

As thrust bearing loads increase, wear and tear and associated noiseincrease at a rate on the order of pressure cubed (i.e., p³ where p ispressure due to the thrust bearing load). This means that a reduction ofpressure on the shaft by two fold translates into a reduction ofassociated wear by eight fold. Such a reduction of pressure on the shaftwould result in a thrust bearing lifetime of eight times its originallifetime. Thus, there is a need for a method and/or apparatus that helpsto reduce thrust bearing loads in homopolar machines.

A unique disadvantage of homopolar machines is that they tend to havelower reliability in comparison to standard DC motors. Specifically,homopolar machines use current collectors to transfer current betweeneach rotating armature turn and each stationary stator turn. Onelimitation of the utility of homopolar machines is the heavy dependenceon current collectors that are potentially unreliable, a large source ofefficiency loss, and maintenance problems.

An important factor in a current collector's performance is its contactpressure with the armature. Performance is measured in terms of currentcollector wear and current carrying capability. Maintaining an idealcontact pressure is difficult because in a homopolar machine the currentcollectors must be in the magnetic field zone where the collectors aresubject to bending and torque. Homopolar machines have been designedwith mechanisms that help to maintain an ideal contact pressure, butthese result in size, weight, and cost penalties, and introduce newsources of reliability problems. Thus, there is a need for a methodand/or apparatus that helps to reduce the wear and tear of currentcollectors in homopolar machines.

SUMMARY OF THE INVENTION

The present invention advantageously addresses the needs above as wellas other needs by providing an apparatus that includes a homopolarmachine. The homopolar machine includes a shaft, an armature assemblycoupled to the shaft, an outer flux return that encloses the armatureassembly, an inner flux return coupled to the shaft, and means forproducing a magnetic field asymmetry within the inner flux return thatproduces an axial force on the shaft.

Another aspect of the present invention provides a homopolar machinecomprising a shaft; a plurality of armature assemblies, coupled to theshaft, that includes a plurality of armature conductor turns; aplurality of current collectors that provide a sliding electricalcurrent interface with the plurality of armature conductor turns; andmeans for maintaining substantially constant contact pressure of thecurrent collectors with the plurality of armature conductor turns in thepresence of high magnetic fields produced by a plurality ofsuperconducting field coils.

And another aspect of the present invention provides a homopolar machinecomprising a shaft; an armature assembly, coupled to the shaft, thatincludes a plurality of armature conductor turns; and a plurality ofstator-current collector arrays that encircle the armature assembly,each stator-current collector array including a plurality of currentcollectors that maintain substantially constant contact pressure withthe armature conductor turns in the presence of a high magnetic field toprovide a sliding electrical current interface with the armatureconductor turns; wherein the plurality of stator-current collectorarrays is oriented such that the high magnetic field is substantiallyparallel to a direction of current flow in a region where at least oneof the plurality of stator-current collectors contacts one of thearmature conductor turns to reduce induced magnetic forces that maydeflect the current collector.

A subsequent aspect of the present invention provides a method ofoperating a homopolar machine, comprising the steps of rotating aplurality of armature assemblies that include a plurality of armatureconductor turns; creating a magnetic field through the plurality ofarmature assemblies; providing a plurality of stator-current collectorarrays that encircle the plurality of armature assemblies, eachstator-current collector array including a plurality of currentcollectors which provide a sliding electrical current interface with thearmature conductor turns; reducing induced magnetic forces that maydeflect the current collectors by directing magnetic field linessubstantially parallel to a direction of current flow in a region whereat least one of the plurality of current collectors contact the armatureconductor turns; and maintaining substantially constant contact pressurebetween the plurality of current collectors and the plurality ofarmature assemblies in the presence of the magnetic field.

An additional aspect of the present invention provides a method ofoperating a homopolar machine, comprising the steps of energizingsuperconducting field coils in the homopolar machine to create amagnetic field through a plurality of armature assemblies, each of theplurality of armature assemblies including a plurality of armatureconductor turns; supplying current to a plurality of current collectorsthat provide a sliding electrical current interface with the pluralityof armature conductor turns; and maintaining an orientation of theplurality of current collectors and the magnetic field so that magneticfield lines are directed substantially parallel to a direction ofcurrent flow in a region where at least one of the plurality of currentcollectors contact the armature conductor turns to reduce inducedmagnetic forces on the plurality of current collectors.

Another aspect of the present invention provides an apparatus thatincludes a homopolar machine, the homopolar machine comprising a shaft;an armature assembly coupled to the shaft; and a plurality ofsuperconducting coils configured to produce a magnetic field asymmetryfor generating an axial force on the shaft to compensate for an externalaxial force imposed on the shaft.

An additional aspect of the present invention provides A method ofoperating a homopolar machine, comprising rotating an armature assemblycoupled to a shaft wherein the shaft rotates with the armature assembly;energizing a first field coil in the homopolar machine to a firstexcitation level to generate a first magnetic field through the armatureassembly coupled to the shaft; and energizing a second field coil in thehomopolar machine to a second excitation level to generate a secondmagnetic field through the armature assembly wherein the secondexcitation level is different than the first excitation level forproducing a magnetic field asymmetry that generates an axial force onthe shaft to compensate an external axial force imposed on the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects featured and advantages of the presentinvention will be more apparent from the following more particulardescription thereof presented in conjunction with the following drawingsherein;

FIG. 1 is a perspective view illustrating the main components of ahomopolar machine with two superconducting coils in accordance with thepresent invention;

FIG. 2 is a top planar cross-sectional view of the homopolar machine ofFIG. 1;

FIG. 3 is a side view illustrating a homopolar machine in accordancewith the present invention having an external thrust bearing;

FIG. 4 is a partial top planar cross-sectional view of the homopolarmachine of FIG. 1 illustrating the pair of field coils energized toequal but opposite polarity amp-turns and the resulting magnetic fields;

FIG. 5 is a partial top planar cross-sectional view of the homopolarmachine of FIG. 1 illustrating the pair of field coils energized todifferent amp-turns relative to each other causing a magnetic fluxasymmetry that produces a compensating axial force in accordance withthe present invention;

FIG. 6 is a partial side view taken along line 6—6 in FIG. 2illustrating part of the stator-current collector assembly and part ofan armature assembly that may be employed in the homopolar machine ofFIG. 1;

FIG. 7 is an end view as seen from line 7—7 in FIG. 6 of part of thestator-current collector assembly and armature shown in FIG. 6;

FIG. 8 is a perspective view illustrating a ten conductor turn armatureassembly that may be used in accordance with the present invention;

FIG. 9 is a close-up perspective view illustrating a portion of the tenconductor turn armature assembly shown in FIG. 8;

FIG. 10 is a close-up perspective view illustrating the stator-currentcollector array shown in FIG. 8;

FIG. 11 is a perspective view illustrating the main components of ahomopolar machine with three superconducting coils in accordance withanother embodiment of the present invention;

FIG. 12 is an exploded perspective view illustrating the main componentsof the homopolar machine of FIG. 11;

FIG. 13 is a top planar cross-sectional view of the homopolar machine ofFIG. 11;

FIG. 14 is a partial cross-sectional view illustrating the magneticfields generated by the superconducting coils of the homopolar machineof FIG. 11;

FIG. 15 is a isometric view of part of the stator for the homopolarmachine of FIG. 11; and

FIG. 16 is a block diagram illustrating a feedback system forcontrolling the compensating force on the shaft in accordance withanother embodiment of the present invention.

Corresponding reference characters indicate corresponding componentsthroughout several figures.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following description of the presently contemplated best mode ofpracticing the invention is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principle of theinvention. The scope of the invention should be determined withreference to the claims.

In accordance with the principles of the present invention, a homopolarmachine is described that addresses the problems discussed above withrespect to thrust bearing wear and current collector performance.

With respect to thrust bearing wear, a homopolar machine in accordancewith the present invention, designed as either a motor or a generator,produces an axial counter force on the rotating shaft to compensate forthe load on the shaft's thrust bearing to reduce wear and noise andprolong bearing life. The counter force relieves the thrust bearing loadand is produced through magnetic interaction between the shaft and themachine's field coils. As will be discussed below, this is accomplishedby positioning part of the machine's iron flux return on the shaft(herein referred to as the inner flux return), which magneticallycouples with the magnetic field produced by the field coils. An axialforce is produced on the shaft by adjusting the magnetic field producedby the field coils by adjusting the current flow through the field coilsto different levels. This can be accomplished without affecting themachine's torque, as long as the ampere-turns change in one coil ismaintained equal and opposite of the ampere-turns change in the othercoil.

As mentioned above, a bearing's wear rate or “life” reducesexponentially with the load or force on the bearing, so even fractionalreductions in bearing load can prolong a bearing's life significantly.An additional benefit of reduced pressure on the thrust bearing is alsoreduced acoustic noise, an important factor in applications that requirequieter running machines, such as submarine propulsion systems.

With respect to current collector performance, a homopolar machine inaccordance with another aspect of the present invention has aconfiguration that uses current collectors that maintain substantiallyconstant contact pressure on the armature surface in the presence ofhigh magnetic fields. This is because the magnetic field lines areoriented in substantially the same direction as the direction of currentflow in the current collectors, so the I×B force that would disturb thecontact pressure is reduced. As will be discussed below, the currentcollectors are flexible and may be made from either electricallyconductive fibers or stacked strips such that they bear up against thearmature so that the pressure is maintained by the spring constant ofthe current collector material.

Referring to FIGS. 1 and 2, there is illustrated a homopolar machine 100in accordance with the present invention. The homopolar machine 100 maybe operated as either a direct current (DC) motor or a generator. Ingeneral, the homopolar machine 100 includes an outer flux return 102, ashaft 104, an armature assembly 106, several stator-current collectorarrays 108, and two field coils 110, 112. The preferred geometry of thehomopolar machine 100 is such that the pair of field coils 110, 112 arecircumferentially shaped and are mounted in line (along a common z-axis)with the armature assembly 106 located between them. The outer fluxreturn 102 substantially surrounds all the components. By way ofexample, the homopolar machine 100 may have a radius of approximately0.5 meters for a poser level of about 5000 HP at 500 RPM.

The outer flux return 102 is preferably substantially cylindrical inshape and is constructed from a material with high magneticpermeability, such as but not limited to, iron or steel. The outer fluxreturn 102 encloses the machine 100's internal components andsubstantially reduces the amount of magnetic flux radiating from themachine. The outer flux return 102 may serve as a substantial structurethat reacts the machine's torsional and magnetic loads and may beaugmented with additional structural reinforcement 114.

The shaft 104 is supported by radial bearings 116 and a thrust bearing118. As shown in FIG. 2, the thrust bearing 118 is internal to thehomopolar machine 100. Alternatively, FIG. 3 illustrates the scenariowhere a thrust bearing 119 is external to a homopolar machine 101, beingpart of an interconnecting drive-train. The thrust bearing 119 maintainsthe axial position of a shaft 105 in the presence of a large externalfor, as for example from a ship propeller.

The armature assembly 106 and the stator-current collector arrays 108are preferably comprised of electrically conductive elements made fromcopper, copper alloy, or other materials with similar electricalproperties. The armature assembly 106 preferably includes a plurality ofstacked armature electrical conducting turns 120, 122, 124, 126, 128separated by electrical insulation 130. The plurality of armatureconducting turns 120, 122, 124, 126, 128 are essentially stackedconcentric cylinders having a squared-U, radial-axial cross-sectionalshape as shown. Each of the conducting turns may be comprised of asingle element or a plurality of elements. In the example shown, thearmature assembly 106 includes five conducting turns 120, 122, 124, 126,128. It should be well understood, however, that the armature assembly106 may include any number of conducting turns. Furthermore, eacharmature conductor turn 120, 122, 124, 126, 128 is preferablydistributed symmetrically around the circumference, therefore resultingin a substantially uniform electric current distribution around thecircumference.

The armature conducting turns 120, 122, 124, 126, 128 conduct current toseveral current collectors 132 and stator conductor turns 134 that areelectrically connected in series between the armature conducting turns120, 122, 124, 126, 128 to form one of the stator-current collectorarrays 108. A plurality of stator-current collector arrays 108 arepreferably distributed circumferentially and symmetrically around thearmature assembly 106 to form the stator. This distribution aids in thedesired circumferentially uniform distribution of current throughout allthe electrically conductive elements of the machine. The uniformity ofcurrent distribution circumferentially around the machine is desirableto prevent thermal hot spots and noise harmonics. The electric currentcarrying elements may be water-cooled via internal cooling channels,cooled by direct spray of a liquid coolant compatible for use withexposed electrical conductors, forced air or internal circulated gas, orany combination of these methods. Such methods would be chosen based onthe requirements and heat dissipation characteristics of the machine.

The field coils 110, 112, which provide the background magnetic field,may be resistive electromagnet coils or superconducting coils. As willbe discussed below, the strength of each of the field coils 110, 112 isvaried, and therefore, permanent magnets are not used for the fieldcoils 110, 112. Preferably, the field coils 110, 112 arecircumferentially continuous in geometry and are superconducting coilsthat are formed from circumferentially wrapped electricallysuperconducting wire. Superconducting coils are the preferred choice dueto the higher magnetic fields that may be produced, which results in asmaller and lighter motor. Superconducting wire provides zero or nearlyzero electrical resistance when cooled to low temperatures. Thisproperty allows more electrical current to flow through the coils ascompared with ordinary wire, such as copper wire, which allows highermagnetic fields to be produced to obtain more powerful machines for agiven size and weight. Thus, superconducting coils do not contributesignificantly to the inefficiency of the machine.

For each of the field coils 110, 112 that is manufactured fromsuperconducting wire, a field coil cryostat 136 is provided that isessentially a vacuum vessel. The vacuum vessel has disposed within itthe field coils 110, 112, which are structurally supported by cryostatcold-to-warm support structures 138. The field coil cryostat 136 will bediscussed in further detail below.

In accordance with the present invention, an inner flux return 140 ispositioned around the inner perimeter of the armature assembly 106. Theinner flux return 140 is preferably cylindrical in shape and isconstructed from a material with high magnetic permeability, such as butnot limited to iron, steel or other ferromagnetic material. The innerflux return 140 is mechanically coupled to the machine shaft 104 via ashaft structure 142 which forms part of the inner flux return and isencircled by the armature assembly 106, i.e., circumferentiallysurrounded by the armature assembly 106. The armature assembly 106 andthe inner flux return 40 both comprise a rotor assembly, which isrotationally disposed within the stator (i.e., the plurality ofstator-current collector arrays 108). The inner flux return 140 plays animportant role in the present invention in that it serves a dual role ofproducing the axial compensating force and directing the magnetic fluxacross the armature assembly 106 and stator to enhance flux coupling forhigher power utilization.

The field coils 110, 112, outer flux return 102, armature assembly 106,stator current collector arrays 108, and inner flux return 140 arepreferably configured in a cylindrical geometry and assembled so theyall share a common central geometric axis 144. The common centralgeometric axis 144 is also coincident with the rotational axis of theshaft 104 and the central magnetic axis of the field coils 110, 112.During operation, the field coils 110, 112 are energized so that theirpolarities are opposite, causing their magnetic flux lines to repel anddeflect into the inner flux return 140 and outer flux return 102, andacross the armature assembly 106 and stator-current collector arrays108. Because the magnetic fields are uniformly distributed in thecircumferential direction, no difference in magnetic flux or multiplemagnetic poles are encountered by conductive elements of the armatureassembly 106 as it rotates, thus functioning as a homopolar machine.

The outer flux return 102 and the inner flux return 140, beingconstructed from high magnetically permeable material, direct themagnetic field generated by the field coils 110, 112 substantiallytowards the region disposed by the armature assembly 106. The fieldlines generated by the field coils 110, 112 are also directed so thatthey are substantially parallel to the outward radial direction ofcurrent flow through the current collectors 132. This reduces any forcesgenerated within the current collectors 132 that arise from current flowthat is non-parallel to the field lines, which reduces current collectordeflections in the circumferential direction. These deflections candisturb the contact pressure between the current collector and thearmature.

When the homopolar machine 100 is operated as a motor, current issupplied to the rotating armature assembly 106 from the stationarystator via the current collectors 132 located between the armature andthe stator. The motor torque is developed as electric current flows atright angles to the flux generated by the stationary field coils 110,112. When the homopolar machine 100 is operated as a generator, themechanically-driven rotating armature assembly 106 generates current asits conductive elements are moved at right angles to the flux, and thecurrent is transported to the stationary stator via the currentcollectors 132.

In accordance with the present invention, the homopolar machine 100 iscapable of producing an axial force on the shaft 104 to compensate forexternal axial forces imposed on the shaft 104. As described above, suchexternal axial forces can be caused, for example, by a connectedpropellor in a ship motor scenario. In general, the compensating axialforce is created by changing the current excitation of the field coils110, 112, which results in an axial magnetic flux asymmetry in the innerflux return 140.

Referring to FIG. 4, when both identical field coils 110, 112 areenergized to the same level of current but with opposite polarity, theyeach produce an equal but opposing magnetic field as shown. The innerflux return 140 deflects the magnetic field lines so that they arefocused more strongly in the armature assembly 106 interaction region.Furthermore, the field lines are oriented substantially parallel to thedirection of the current flow in the stator-current collector arrays 108region (radially outward). This orientation reduces the force exerted onthe current collectors 132 in the circumferential direction by reducingthe current and field line (I×B) force. By way of example, theillustrated homopolar machine 100 may have an outer diameter of 1 meterand be capable of producing 5000 HP at 500 rpm, and in this particularexample, the field coils 110, 112 are both energized to produce 1.5×10⁶Amp-turns.

Referring to FIG. 5, by altering the current flow in the field coils110, 112 so the current flows (or amp-turns) are different from eachother, the homopolar machine 100 produces a compensating axial force 141on the shaft 104. When the field coils 110, 112 are energized todifferent amp-turns levels relative to each other, an axial fluxasymmetry is caused that produces the axial force 141 on the inner fluxreturn 140 mounted to the shaft 104. External axial forces 143 imposedon the shaft 104 are compensated by the force 141 generated in the innerflux return 140 by the asymmetric magnetic flux. The direction of thecompensating axial force 141 is selected to be opposite to that of thethrust bearing load. The force 141 is reacted by the thrust bearing 118,and also by the field coils 110, 112, and into the outer flux return 102through various interconnecting elements including support structure138. The compensating axial force 141 reduces the net force on thethrust bearing which helps to reduce thrust bearing wear and increasesthrust bearing reliability and reduces noise generated by the thrustbearing.

FIG. 5 illustrates the intended effect where the current in one fieldcoil 110 is raised by 10% and the current in the other field coil 112 islowered by 10%. In this particular example, the first field coil 110 isenergized to 1.35×10⁶ Amp-turns (Peak Field=4.9 Tesla) and the secondfield coil 112 is energized to 1.65×10⁶ Amp-turns (Peak Field=6 Tesla).The peak magnetic field in the stronger coil 112 is about 6 Tesla, andthe average coil current density in it is about 10 kA/cm². These arereasonable parameters for a low temperature superconducting coil. Inthis case, the axial force 141 induced on the shaft 104 by the fluxasymmetry is predicted to be 20,000 lbs.

The current in the field coils 110, 112 can be altered to produce thiscompensating force 141, while the total integrated magnetic field-lengthtimes current product through the armature remains constant. This meansthat the rotational torque, and therefore the rotational speed of thehomopolar machine 100, need not be interrupted.

Thus, the stationary electromagnetic field coils 110, 112 produce allthe magnetic flux necessary for the machine to develop power, and asubstantial fraction of this flux is coupled to the inner flux return140 that is positioned between the field coils 110, 112 and mounted onthe rotating shaft 104. An asymmetry in the flux distribution is createdby energizing the field coils 110, 112 to different levels, whichresults in an induced axial force 141 on the rotating shaft 104. Theasymmetric axial flux that is created by the field coils 110, 112 isproduced by raising the excitation of one field coil and lowering theexcitation of the other field coil approximately by the same amount, sothe total flux across the armature and stator interaction region remainsconstant or substantially constant. The current through the armature maybe adjusted as necessary to allow the shaft 104's induced axial force141 component to be varied while the shaft's rotational torque is notaffected. The direction of the induced axial force 141 is selected tocompensate for mechanical thrust induced on the shaft 104.

The present invention does not rely on additional auxiliary coils sincethe main field coil pair 110, 112 performs the dual function ofproducing magnetic flux for rotational torque and axial thrust.Furthermore, it is not necessary to vary the mechanical positioningoffset of the shaft 104 relative to the field coil alignment. Thehomopolar machine 100's magnetic flux path is also designed to reducemagnetically induced forces that may upset the mechanical pressureexerted by the current collectors 132 on the armature assembly 106.

Referring to FIG. 6, there is illustrated a top view, looking down onthe surface of the rotor, of one complete stator-current collector array158 and part of the armature assembly 106. The illustrated example isfor an armature assembly 106 with five armature conductor turns 120,122, 124, 126, 128. Again, it should be well understood that thearmature assembly 106 may have a different number of armature conductorturns. The stator-current collector array 158 includes stator conductorturns 146, 148, 150, 152, 154, 156, 159, 160, 162, 164, 165, 167.Several stator-current collector arrays 108, like the array 158, arearranged circumferentially around the homopolar machine 100 todistribute the current.

FIG. 7 illustrates an end view of a portion of the stator-currentcollector array 158 and the armature conductor turn 120. The statorconductor turns 148, 160 are electrically connected to theircorresponding circumferential busbar 166, while the other illustratedstator conductor turns 146, 150, 152, 154, 156, 162 are electricallyinsulated from the busbar 166 with electrical stator turn-to-turninsulation 168. The stator conductor turns 146, 150, 152, 154, 156, 162are electrically insulated from the busbar 166 because they correspondto busbars associated with other circumferential busbars armatureconductor turns. Specifically, the stator conductor turn 150 correspondsto a bus bar (not shown) associated with the armature conductor turn122, the stator conductor turn 152 corresponds to a bus bar (not shown)associated with the armature conductor turn 124, the stator conductorturn 154 corresponds to a bus bar (not shown) associated with thearmature conductor turn 126, and the stator conductor turn 156corresponds to a bus bar (not shown) associated with the armatureconductor turn 128. In the illustrated embodiment, pairs of stator turnsconnect to a common busbar. For example, the stator conductor turns 148and 160 are electrically in parallel and connect to the busbar 166.

The busbar 166 includes five current collectors 132 that conduct currentto or from the armature conductor turn 20. It should be well understood,however, that any number of current collectors 132 may be attached tothe busbar 166.

Referring to FIGS. 6 and 7 together, DC electric current is routedthrough the stator terminals 170, 171, 172, 173 such that current flowsbetween the positive (+) terminals 170, 171 and negative (−) terminals172, 173. Specifically, current that enters the positive (+) terminals170, 171 flows through the stator conductor turns 146, 159, then througha circumferential busbar (not shown), then through a current collector(not shown), then through the armature conductor turn 120, then throughthe current collectors 132, then through the circumferential bus bar166, then through the stator conductor turns 148, 160, and continues ina similar manner through the remaining armature conductor turns 122,124, 126, 128, and out the negative (−) terminals 172, 173.

In accordance with the present invention, the current collectors 132comprise a flexible electrically conductive material, such as but notlimited to, electrically conductive fibers or foils made from copper orcopper alloys or stacked strips. The current collectors 132 areconfigured to exhibit a spring rate that results in an applied pressureon the armature conductor turn 120 when connected to the circumferentialbusbar 166. Furthermore, the current collectors 132 are configured to befree to bear up against the outer edge of the armature turn 120's smoothsurface. Thus, the current collectors 132 are flexible such that theybear up against the armature so that the pressure is maintained by thespring constant of the current collector material or by a holder thatprovides a mechanical and electrical component between the currentcollectors 132 and circumferential busbar 166.

As the current collectors 132 wear, the pressure will be relaxed onlypartially because each current collector is flexible owing to its springconstant or the holder accommodates the wear of the brush material. Thecurrent collector pressure, however, can be undesirably affected byforces generated on it due to current flowing through it in the presenceof a magnetic field. In the present invention, the flux return geometryand field coil configuration are designed so that the magnetic fieldlines are substantially in the same direction as the direction of thecurrent flow through the current collectors 132, i.e., radially outward.Since the current flowing through the current collectors passessubstantially parallel to the field lines, minimal force (I×B) isinduced on the flexible collectors 132. The force is minimal enough suchthat the flexible collectors 132 are not deflected enough to change thecontact pressure enough to significantly degrade performance.

The wear rate, electrical resistance losses, and friction losses at thecurrent collector 132 and armature conductor turn 120 interface can bemitigated by the presence of a controlled atmospheric chemicalcomposition within the machine enclosure, or by the addition of moisturein the armature region in the form of water humidity or direct wettingwith a liquid compatible for use with exposed electrical conductors.

As mentioned above, the armature assembly 106 may have any number ofarmature conductor turns, and there may be any number of currentcollectors 132 attached to the busbar 166 with or without interfacingholders for the current collectors 132. By way of example, FIG. 8illustrates an armature assembly 200 and a stator-current collectorarray 202 that may be used in accordance with the present invention.Although the armature assembly 200 would typically be encircled byseveral stator-current collector arrays, only the stator-currentcollector array 202 is illustrated.

The armature assembly 200 includes ten armature conductor turns 206,208, 210, 212, 214, 216, 218, 220, 222, 224. The front portion of thestator-current collector array 202 includes stator conductor turns 226,228, 230, 232, 234, 236, 238, 240, 242, 244, 246, and the terminals 248,250. Referring to FIG. 9, the stator conductor turns 226, 228, 230, 232,234, 236, 238, 240, 242, 244, 246 are electrically coupled to thebusbars 252, 254, 256, 258, 260, 262, 264, 266, 268, 270, 272,respectively. The stator conductor turns 228, 230, 232, 234, 236, 238,240, 242, 244, 246 are also electrically coupled to the busbars 274,287, 285, 283, 281, 279, 277, 275, 273, 272, respectively. Furthermore,the stator conductor turns 228, 276 are electrically coupled to thebusbars 274 and 254.

Referring to FIG. 10, each of the busbars includes ten currentcollectors attached thereto. For example, the busbar 274 includes theten current collectors 278, 280, 282, 284, 286, 288, 290, 292, 294, 296,and the busbar 252 includes the ten current collectors 298, 300, 302,304, 306, 308, 310, 312, 314, 316. Thus, any number of currentcollectors may be attached to the busbars with or without interfacingholders in accordance with the present invention. The current collectorsare attached to the busbars by means of brush mounts (also referred toas brush holders or brush mount interfaces). For example, the currentcollector 298 is attached to the busbar 252 by means of the brush mount320, and the current collector 278 is attached to the busbar 274 bymeans of the brush mount 322.

With respect to the field coils 110, 112, superconducting coils aretypically constructed by winding superconducting wire. They may becategorized as low temperature superconductors, which typically mustoperate in the range of 4 K to 12 K, or high temperaturesuperconductors, which may be capable of operating at temperatures up to80 K. Typical materials for low temperature superconductors includedniobium titanium (NbTi) or niobium and tin (Nb₃Sn), both of which aretypically formed in a matrix with copper or copper alloys to form wire.High temperature superconductors use other constituents, such as BSCCO(Bismuth Spartium Carbon Copper Oxide) or YBCO (Yttrium Barium CopperOxide) formed in a metal matrix, typically silver.

Superconducting coils, either high or low temperature, are typicallystructurally suspended within a cryostat. A cryostat is comprised of avacuum vessel and a support structure that suspends the coil within itso reduced heat flows into the coil from the structure. The supportstructure itself is typically strong enough to support the coil againstvarious forces that may be present. The field coils 110, 112 may becooled within the cryostat 136 by a cryogenic fluid such as liquidhelium which typically operates at a temperature of 4.2 K, or othercryogenic fluids can be used such as liquid nitrogen at 77 K, if thesuperconducting coil is designed to operate at that temperature. Thefield coils 110, 112 may also be cooled by devices known as cryocoolers,which are closed cycle cryogenic refrigeration devices that can lowerthe field coil's temperature to the required level.

With respect to the field coil cryostat 136, the cryostat cold-to-warmsupport structure 138 is preferably comprised of materials strong enoughto physically support the field coil 110, 112 while reducing the amountof heat that can flow to it from the vacuum vessel, which is at roomtemperature. Because they are superconducting coils, the field coils110, 112 are at a much colder temperature, typically in the range of 4 Kto 12 K for low temperature superconductors and up to 80 K for hightemperature superconductors.

Each of the field coils 110, 112 may be cooled by connecting them todevices called “cryocoolers” via thermally conducting links (not shown).The cryocoolers are generally part of a closed cycle refrigerationsystem. Another method that can be used to cool the field coils 110, 112to required temperature makes use of another vessel (not shown) disposedbetween the coil and the vacuum vessel. The vessel is filled with aliquid cryogen, such as but not limited to liquid helium, liquidhydrogen, or liquid nitrogen, depending on the temperature requirementsof the superconductor material used in the field coils 110, 112. Ineither method, a cryostat thermal shield 174, comprising a thermallyconductive material such as but not limited to copper or aluminum, ispreferably disposed between the cryostat vacuum vessels and the fieldcoils 110, 112. The cryostat thermal shield 174 is attached to thecryostat cold-to-warm support structure 138 to intercept heat conductedand radiated inward from the cryostat vacuum vessel walls and the fieldcoil current leads 176 that would otherwise leak into the field coils110, 112.

The heat is intercepted at thermally conductive mechanical connections178 between the cryostat cold-to-warm support structure 138 and thecryostat thermal shield 174, and also at an electrically insulated butthermally conductive mechanical connection 180 between the field coilcurrent leads 176 and the cryostat thermal shield 174. The interceptedheat is removed by the cryocooler (not shown) which is connected to thethermal shield 174, or in the case where a second inner vessel filledwith liquid cryogen is used (not shown), the heat may be removed throughconduction of the cryogen's boil-off gases that would be routed througha thermally conductive material attached to the cryostat thermal shield174. The field coil cryostats 136 are mechanically supported by theouter flux return 102 via the cryostat mounts 182.

FIG. 11 is a perspective view illustrating the main components of ahomopolar machine in accordance with another embodiment of the presentinvention. Shown is a drive shaft 302, a torque tube 304, a cryostatvacuum vessel 306, a first bearing 308, a second bearing 309, a firstsuperconducting coil 310, a second superconducting coil 312, a thirdsuperconducting coil 314, a plurality of superconducting coil supports316, a first active armature 318, a second active armature 320, a firststator module 322, a second stator module 324, a first plurality ofbrushes 326, a second plurality of brushes 327, a first end cap 328, anda second end cap 330.

The homopolar machine shown in FIGS. 11-13 functions similarly in manyaspects to the homopolar machine shown in FIG. 1. The homopolar machineof FIGS. 11-13 has been modified to incorporate an additionalsuperconducting coil and an additional armature. The present inventioncan also incorporate additional superconducting coils and additionalarmatures as needed without deviating from the scope of the presentinvention.

Advantageously, the embodiment shown in FIGS. 11-13 provide for ahomopolar machine in accordance with the present invention which has anincreased overall power and an increased torque density. In thisembodiment, the first active armature 318 and the second active armature320 are both coupled to the torque tube 304. The torque tube 304 iscoupled to the drive shaft 302 of the homopolar machine.

The first superconducting coil 310, the second superconducting coil 312,and the third superconducting coil 314 are mounted within the cryostatvacuum vessel 306. The cryostat vacuum vessel 306 is stationary andprovides central access for current leads and cryocooler, which coolsthe superconducting coils 310, 312, 314. In one embodiment the cryostatvacuum vessel 306 can be stainless steel and the superconducting coils310, 312, 314 can be made from NbTi (Niobium Titanium). It should beunderstood that other materials can be used for the cryostat vacuumvessel 306 and the superconducting coils 310, 312, 314 without deviatingfrom the scope of the present invention.

The first superconducting coil 310, the second superconducting coil 312,and the third superconducting coil 314 are conduction cooled via aninterface with a cryocooler (not shown). The first superconducting coil310, the second superconducting coil 312, the third superconducting coil314 are thermally isolated from the room temperature outer wall of thecryostat vacuum vessel 306 by being mounted on the plurality ofsuperconducting coil supports 316. The plurality of superconducting coilsupports 316 is formed from composite materials, however, many materialsmay be used in accordance with the present invention. Additionally, anintermediate temperature thermal shield, which is a series of nestedcylinders and cones is located between the cold coils and the warm outershell of the cryostat vacuum vessel 316 to reduce the heat to thesuperconducting coils 310, 312, 314.

The first active armature 318 and the second active armature 320 aremounted on the outside of the torque tube 304. The torque tube 304 isthen coupled to the drive shaft 302, such that the torque tube 304rotates with the drive shaft 302. The first active armature 318 and thesecond active armature 320 both include an inner flux return and anarmature assembly. The first active armature 318 and the second activearmature 320 interface with the first plurality of brushes 326 and thesecond plurality of brushes 327 mounted on the brush holders. The brushholders are coupled to the stator bars which are in 8 pairs of statorsegments. It should be understood that a different number of pairs ofstator segments can be used without deviating from the scope of thepresent invention. The configuration of the stator bars is shown ingreater detail with reference to FIG. 15.

As with the embodiment shown in FIG. 1, the first superconducting coil310, the second superconducting coil 312, and the third superconductingcoil 314 are positioned such that the Lorentz forces (I×B) on the firstplurality of brushes 326 and the second plurality of brushes 327 issubstantially parallel to the current flow which reduces the Lorenzforces on the brushes. The size, shape, and location of thesuperconducting coils and any magnetically permeable elements areadjusted such that the magnetic field in the area of the plurality ofbrushes is aligned as much as is practical with the current flowing inthe plurality of brushes. This reduces the vector cross product (I×B)and, in turn, reduces the forces on the brushes and the brush holders.

In this configuration the brushes maintain substantially constantcontact pressure with either the first active armature 318 or the secondactive armature 320 in the presence of the high magnetic fields createdby the first superconducting coil 310, the second superconducting coil312 and the third superconducting coil 314. Because the magnetic fieldlines are oriented in the same general direction as the direction ofcurrent flow as much as is practical, the I×B force that would disturbthe contact pressure is reduced. The first plurality of brushes 326 andthe second plurality of brushes 327 are flexible and may be made fromeither electrically conductive fibers or stacked strips such that theybear up against the first active armature 318 and the second activearmature 320, respectively, such that the pressure is also maintained bythe spring constant of the brushes or the brush holders.

FIG. 12 is an exploded perspective view of the homopolar machine of FIG.11. Additionally shown in FIG. 12 is a support frame structure 332. Thesupport frame structure 332 provides support for the homopolar machine.The support frame structure 332 is located in between the pairs ofstator modules and in between the first active armature 318 and thesecond active armature 320.

FIG. 13 is a cross-sectional view illustrating the homopolar machine ofFIG. 11. Shown is the drive shaft 302, the torque tube 304, the cryostatvacuum vessel 306, the first bearings 308, the second bearing 309, thefirst superconducting coil 310, the second superconducting coil 312, thethird superconducting coil 314, the plurality of superconducting coilsupports 316, the first active armature 318, the second active armature320, the first stator module 322, the second stator module 324, thefirst plurality of brushes 326, the second plurality of brushes 327, thefirst end cap 328, and the second end cap 330.

The first stator module 322 and the second stator module 324 help directthe magnetic field generated by the first superconducting coil 310, thesecond superconducting coil 312, and the third superconducting coil 314such that the Lorentz forces (I×B) on the first plurality of brushes 326and the second plurality of brushes 327 are reduced. Reduction in theLorentz forces on the brushes helps to maintain a constant contactpressure between the brushes and the active armatures.

FIG. 14 is a partial cross sectional view illustrating magnetic fieldlines in accordance with the homopolar machine of FIG. 11. Shown is aplurality of regions for brushes 600, one of a plurality of magneticfield lines 602, a machine axis 604, a plurality of superconductingcoils 606, a plane of symmetry 608 and a housing 610. Shown is across-section of ¼ of the entire homopolar machine of FIG. 11.

The housing 610 supports the homopolar machine and aids in shaping themagnetic field created by the plurality of superconducting coils 606.The magnetic field is represented by the plurality of magnetic fieldlines 602. The housing 610 helps to direct the magnetic field such thatthe lateral electromagnetic forces on the brushes of the homopolarmachine are reduced. The housing 610 is typically made of iron, however,other materials which help direct the magnetic fields lines can be used.

The plurality of regions for the brushes 600 show approximately wherethe brushes of the homopolar machine are located. The brushes arelocated and oriented relative to the field lines such that optimizationof the torque generation on the rotating parts is achieved.Additionally, the brushes are oriented so that the current is aligned asmuch as is practical with the local magnetic field lines so as to reducethe lateral electromagnetic forces on the brushes. As previouslydescribed herein, the lower the I×B force on the brushes, the more aconstant contact pressure is maintained between the brushes and thearmature. In one embodiment, the magnetic field is entirely parallel tothe plurality of brushes, such that there in no I×B force on thebrushes. In another embodiment, the orientation of the brushes will besuch that there are some magnetic field forces acting on the brushes,however, the brushes are oriented so the magnetic field forces arereduced as much as is practical.

FIG. 15 is a perspective view illustrating the stator bars connected toa pair of stator segments. Shown is a first stator segment 400, a secondstator segment 402, a plurality of stator bars 404 and arrows 406illustrating the direction of current flow.

The plurality of stator bars 404 are coupled to the first stator segment400 and the second stator segment 402 as shown. The configuration shownin FIG. 14 illustrates one manner which allows current flow to betransferred properly among the turns in the first active armature 318and the second active armature 320. Five turns are shown forillustrative purposes, however, for 25 MW and 36.5 MW motors,approximately 15 and 17 turns can be used, respectively. It should beunderstood that the number of turns can vary while not deviating fromthe present invention.

The plurality of stator bars 404 are additionally coupled to brushholders (not shown). The brush holders provide mounting for the brushes.The brushes provide an electrical connection to the first activearmature 318 and the second active armature 320.

As describe above with reference to FIGS. 4 and 5, in accordance withthe present invention, the homopolar machine 100 is capable of producingan axial force on the shaft 104 to compensate for external axial forcesimposed on the shaft 104. External axial forces can be caused, forexample, by a connected propellor in a ship motor scenario. In general,the compensating axial force is created by changing the currentexcitation of the field coils 110, 112, which results in an axialmagnetic flux asymmetry in the inner flux return 140. Similarly, withrespect to the embodiment shown in FIGS. 11-13, the homopolar machine iscapable of producing an axial force on the drive shaft 302. Thecompensating force is created by changing the current excitation of thesuperconducting coils 310, 312, 314, which results in an axial magneticflux asymmetry on the inner flux return of one or both of the firstactive armature 318 and the second active armature 320. The first activearmature 318 and the second active armature 320 are coupled to thetorque tube 304 which is coupled to the drive shaft 302. Thus, thecompensating force on the inner flux return is transferred to the driveshaft 302.

FIG. 16 is a block diagram illustrating a feedback system forcontrolling the compensating axial force in accordance with the presentinvention. Shown is an armature block 500, a superconducting coil block502, a voltage and current sensor block 504, and an active control block506.

The system allows for active control of the armature current and thearmature voltage as well as active control of the currents in thesuperconducting coils. The torque produced by the homopolar machineshown in either FIG. 1 or FIG. 11 is dependent upon the armature currentand the field level produced by the superconducting coils. The voltagerequired by the homopolar machine is determined by the operating speedof the homopolar machine and the field levels created by thesuperconducting coils. The active control block 506 thus provides forthe ability to unload the thrust bearing by adjusting the relativemagnitude of the current in the plurality of superconducting coils. Thevoltage and current sensor block 504, thus provides for the monitoringof the armature current and the armature voltage. Active control of thecurrents in the superconducting coils and the currents and voltages inthe armatures allow the homopolar machine to operate with a reducedaxial load condition on the thrust bearing in an automated and real timefashion while maintaining the desired torque and RPM.

While the invention herein disclosed has been described by the specificembodiments and applications thereof, numerous modifications andvariations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

1. A homopolar machine comprising: a shaft; a plurality of armatureassemblies, coupled to the shaft, that includes a plurality of armatureconductor turns; a plurality of current collectors that provide asliding electrical current interface with the plurality of armatureconductor turns; and means for maintaining substantially constantcontact pressure of the current collectors with the plurality ofarmature conductor turns in the presence of high magnetic fieldsproduced by a plurality of superconducting field coils.
 2. A homopolarmachine in accordance with claim 1 further comprising a torque tubecoupled to the shaft and the plurality of armature assemblies.
 3. Ahomopolar machine in accordance with claim 1 further comprising acryostat vessel for housing a first superconducting field, a secondsuperconducting field coil and a third superconducting field coil.
 4. Ahomopolar machine in accordance with claim 1 further comprising a firstsuperconducting field, a second superconducting field coil and a thirdsuperconducting field coil.
 5. A homopolar machine comprising: a shaft;an armature assembly, coupled to the shaft, that includes a plurality ofarmature conductor turns; and a plurality of stator-current collectorarrays that encircle the armature assembly, each stator-currentcollector array including a plurality of current collectors thatmaintain substantially constant contact pressure with the armatureconductor turns in the presence of a high magnetic field to provide asliding electrical current interface with the armature conductor turns;wherein the plurality of stator-current collector arrays is orientedsuch that the high magnetic field is substantially parallel to adirection of current flow in a region where at least one of theplurality of stator-current collectors contacts one of the armatureconductor turns to reduce induced magnetic forces that may deflect thecurrent collector.
 6. A homopolar machine in accordance with claim 5further comprising a second armature assembly coupled to the shaft.
 7. Ahomopolar machine in accordance with claim 6 further comprising: atorque tube coupled to the shaft; and a first superconducting fieldcoil, a second superconducting field coil, and a third superconductingfield coil enclosed within the torque tube.
 8. A homopolar machine inaccordance with claim 5 further comprising an outer flux return thatencloses the plurality of armature assemblies.
 9. A method of operatinga homopolar machine, comprising the steps of: rotating a plurality ofarmature assemblies that include a plurality of armature conductorturns; creating a magnetic field through the plurality of armatureassemblies; providing a plurality of stator-current collector arraysthat encircle the plurality of armature assemblies, each stator-currentcollector array including a plurality of current collectors whichprovide a sliding electrical current interface with the armatureconductor turns; reducing induced magnetic forces that may deflect thecurrent collectors by directing magnetic field lines substantiallyparallel to a direction of current flow in a region where at least oneof the plurality of current collectors contact the armature conductorturns; and maintaining substantially constant contact pressure betweenthe plurality of current collectors and the plurality of armatureassemblies in the presence of the magnetic field.
 10. A method inaccordance with claim 9, wherein the step of creating a magnetic fieldthrough the plurality of armature assemblies comprises the step ofcreating a magnetic field through the plurality of armature assemblieswith three superconducting field coils.
 11. A method in accordance withclaim 9, wherein each of the current collectors comprises a solidmaterial.
 12. A method in accordance with claim 9, wherein each of thecurrent collectors comprises a flexible, electrically conductivematerial.
 13. A method in accordance with claim 9, wherein each of thecurrent collectors comprises electrically conductive fibers made fromcopper.
 14. A method in accordance with claim 9, wherein each of thecurrent collectors comprises electrically conductive fibers made fromcopper alloys.
 15. A method in accordance with claim 9, wherein each ofthe current collectors comprises electrically conductive foils made fromcopper.
 16. A method in accordance with claim 9, wherein each of thecurrent collectors comprises electrically conductive foils made fromcopper alloys.
 17. A method of operating a homopolar machine, comprisingthe steps of: energizing superconducting field coils in the homopolarmachine to create a magnetic field through a plurality of armatureassemblies, each of the plurality of armature assemblies including aplurality of armature conductor turns; supplying current to a pluralityof current collectors that provide a sliding electrical currentinterface with the plurality of armature conductor turns; andmaintaining an orientation of the plurality of current collectors andthe magnetic field so that magnetic field lines are directedsubstantially parallel to a direction of current flow in a region whereat least one of the plurality of current collectors contact the armatureconductor turns to reduce induced magnetic forces on the plurality ofcurrent collectors.
 18. A method in accordance with claim 17, whereineach of the current collectors comprises a solid material.
 19. A methodin accordance with claim 17, wherein each of the current collectorscomprises a flexible, electrically conductive material.
 20. An apparatusthat includes a homopolar machine, the homopolar machine comprising: ashaft: an armature assembly coupled to the shaft; and a plurality ofsuperconducting coils configured to produce a magnetic field asymmetryfor generating an axial force on the shaft to compensate for an externalaxial force imposed on the shaft.
 21. An apparatus in accordance withclaim 20 further comprising a feedback system for controlling themagnetic field and the current and voltage in the armature assembly. 22.A homopolar machine in accordance with claim 20, further comprising anouter flux return comprising a geometry that directs magnetic fieldlines substantially parallel to a direction of current flow in a regionwhere a plurality of current collectors contact the armature assembly toreduce induced magnetic force that may deflect the current collectors ina circumferential direction.
 23. A homopolar machine in accordance withclaim 22, wherein each of the current collectors comprises a flexible,solid material that is coupled to a respective stator-current collectorarray so that it bears up against an outer rotating rim of the armatureassembly.
 24. A homopolar machine in accordance with claim 22, whereineach of the current collectors comprises a flexible, solid electricallyconductive material.
 25. A homopolar machine in accordance with claim22, wherein each of the current collectors comprises electricallyconductive fibers made from copper.
 26. A homopolar machine inaccordance with claim 22, wherein each of the current collectorscomprises electrically conductive fibers made from copper alloys.
 27. Ahomopolar machine in accordance with claim 22, wherein each of thecurrent collectors comprises electrically conductive foils made fromcopper.
 28. A homopolar machine in accordance with claim 22, whereineach of the current collectors comprises electrically conductive foilsmade from copper alloys.
 29. A method of operating a homopolar machinecomprising: rotating an armature assembly coupled to a shaft wherein theshaft rotates with the armature assembly; energizing a first field coilin the homopolar machine to a first excitation level to generate a firstmagnetic field through the armature assembly coupled to the shaft; andenergizing a second field coil in the homopolar machine to a secondexcitation level to generate a second magnetic field through thearmature assembly wherein the second excitation level is different thanthe first excitation level for producing a magnetic field asymmetry thatgenerates an axial force on the shaft to compensate an external axialforce imposed on the shaft.
 30. A method of claim 29 further comprisingadjusting the excitation level in one of the first field coil and thesecond field coil in response to a feedback signal.
 31. A method ofclaim 29 further comprising energizing a third field coil in thehomopolar machine to a third excitation level to generate secondmagnetic field though a second armature assembly coupled to the shaft.32. A method of claim 29 further comprising measuring one of thearmature current and the armature voltage to determine the feedbacksignal.
 33. A method of claim 29 further comprising rotating an secondarmature assembly coupled to the shaft wherein the shaft rotates withthe second armature assembly.